Re: [Xen-devel] [PATCH RFC 3/4] Arm64: further speed-up to hweight{32, 64}()

[Julien Grall <julien.grall@xxxxxxx>]

Hi Jan,

On 5/31/19 10:53 AM, Jan Beulich wrote:
> According to Linux commit e75bef2a4f ("arm64: Select
> ARCH_HAS_FAST_MULTIPLIER") this is a further improvement over the
> variant using only bitwise operations on at least some hardware, and no
> worse on other.
>
> Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> RFC: To be honest I'm not fully convinced this is a win in particular in
>       the hweight32() case, as there's no actual shift insn which gets
>       replaced by the multiplication. Even for hweight64() the compiler
>       could emit better code and avoid the explicit shift by 32 (which it
>       emits at least for me).
I can see multiplication instruction used in both hweight32() and 
hweight64() with the compiler I am using.
I would expect the compiler could easily replace a multiply by a series 
of shift but it would be more difficult to do the invert.
Also, this has been in Linux for a year now, so I am assuming Linux 
folks are happy with changes (CCing Robin just in case I missed 
anything). Therefore I am happy to give it a go on Xen as well.
Cheers,

> --- a/xen/arch/arm/Kconfig
> +++ b/xen/arch/arm/Kconfig
> @@ -12,6 +12,7 @@ config ARM_32
>   config ARM_64
>         def_bool y
>         depends on 64BIT
> +       select HAS_FAST_MULTIPLY
>   
>   config ARM
>         def_bool y
--
Julien Grall

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